1. Field of the Invention
This invention relates to a digital communication bus for connecting electronic modules.
2. Discussion of the Background
Automation functions (racks, modules and various couplers) can be placed in different locations if a bus B is used to connect the various automation stations such as S1, S2, Sn, as shown in FIG. 1. This bus consists of transmission pairs and is terminated at both ends by identical line terminators TA and TB. This layout makes it impossible to detect a bus continuity fault.
Patent EP 0 517 609 describes a serial bus capable of connecting this type of automation module.
The purpose of this invention is to provide a bus capable of detecting a bus continuity fault, for example a connection fault in one of the two line terminators. If there is no fault, the line used for this detection can be used for digital communications.
The bus according to the invention is composed of at least one pair of data transmission lines and terminates with line terminators and is characterized essentially by the fact that it comprises components, for example in the two line terminators, imposing transmission with polarization and inverting the polarization whenever at least one of the line terminators is missing, or is not connected to the bus, or the bus is open.
According to one characteristic, the bus comprises two resistances in each module, associated with a transmission pair and capable of polarizing this pair, one of these resistances being connected to a first line in the said pair, the other resistance being connected to the second line.
According to another characteristic, the bus comprises a dividing bridge at each end, the mid-point of this bridge being connected to one line, and the second line being connected to a resistance at the first line terminator end, the first line being connected to a resistance at the second line terminator end, the bridges being inverted at one end compared with the other.
According to another characteristic, at each end the bus includes a doubly dividing bridge with three resistances, the first line being connected at the first line terminator end to a capacitor connected to one of the two mid-points of the bridge, the second line being connected at the second line terminator end to a capacitor connected to one of the two mid-points of the bridge.